There have been conventional decarburizing methods of high Cr molten metal known as AOD, VOD, VODC or RH-OB processes. In AOD, a mixed gas of oxygen and an inert gas from a furnace bottom is blown under an atmospheric pressure. In VOD and VODC, O.sub.2 is blown under a vacuum, and in RH-OB, a rough decarburization is performed in a converter, and subsequently a vacuum O.sub.2 blowing is carried out in a RH degasfying chamber.
However, since in the AOD process, the decarburization is carried out in the air and O.sub.2 is blown directly into the molten metal, loss of Cr by its oxidation (called as "Cr oxidation loss" hereinafter) is large, and it is disadvantageous that much Fe--Si or Al must be thrown into a slag as a reducing agent.
In the VOD, VODC and RH-OB processes, the decarburization is undertaken in the vacuum with less Cr oxidation loss, but since much oxygen is necessary for securing productivity, a vacuum facility of a large capacity therefor is required at high cost.
Also when the high Cr molten metal is decarburized by blowing in the air, the inert gas is much required to agitate the molten metal. Ordinarily available gases therefor are N.sub.2 and Ar. If a low nitrogen stainless steel is produced, N.sub.2 cannot be used because it increases N concentration in steel, and an expensive Ar must be used. If Ar were much used, a problem would arise about the production cost. From such a viewpoint, the production of the low nitrogen steel usually depends upon a vacuum blowing, by which the denitrification takes place easily.
For these foregoing problems, the present invention has been devised. It is a basic object of the invention to provide a method which could perform the decarburization in a short period of time without practising the vacuum decarburization and with checking Cr oxidation loss.
It is the other object of the invention to provide a method which could decrease N in steel, not using much Ar, thereby to enable production of low nitrogen stainless steel at low production cost.